The present invention relates to a method of measuring thickness of liquid crystal cells.
Liquid crystals are used in a variety of display apparatus. When unevenness takes place on the display of liquid crystals, their performance of a display apparatus is deteriorated. One of the causes which contribute to such display unevenness is nonuniformity in thickness of the liquid crystal cells. Attempts have been made to develop methods of accurately measuring the thickness of the liquid crystal cells. For example, Japanese Unexamined Patent Publication 2-118406 has disclosed a method for measuring the thickness of a liquid crystal cell, based on retardation R, between ordinary ray and extraordinary ray, experimentally obtained according to the characteristic that liquid crystals show optical anisotropy. Specifically, interference fringes are generated under crossed-Nicol conditions, and the strength of output light is measured varying the wavelength of the light. Wavelength .lambda..sub.0 corresponding to an order number m which produces a maximum or a minimum strength of output light is obtained, and then retardation R is determined by the following equation: EQU R/.lambda.0=m
Further, in the method disclosed, since difference .DELTA.n between an index of refraction no of the ordinary ray and an index of refraction of ne of the extraordinary ray is a known value, the thickness d according to the retardation R is determined as follows: EQU R=.DELTA.n.multidot.d
Although the above-mentioned method can be applied to ordinary optical anisotropic crystals, it cannot be applied to a liquid crystal, such as a twisted nematic liquid crystal (referred to as TN liquid crystal, hereinafter), in which the orientation of the molecules varies spatially.
An equation describing the propagation of light in the TN liquid crystal has been found. Reference is made to H. L. Ong, Appl. Phys. Lett. 51(18), Nov. 2, 1987, pp 1398-1400. Stated in this reference is an equation (equation (3) in the reference), into which the orientation of an analyzer, the orientation of a polarizer, an index of refraction of a liquid crystal, a twist angle, the angle of liquid crystal molecules with respect to a base plate (pre-tilt angle) and a cell thickness are all factored in a generalized manner.
It is an object of the present invention to provide a method of accurate thickness measurement in a TN liquid crystal, by applying the above-mentioned equation which describes the propagation of light in the TN liquid crystal.